Chemistry 121/122 Acids, Bases and Salts

Properties of Acids Acids, in foods, give a sour or tart taste Aqueous solutions of acids are electrolytes (conduct electricity (release ions in solution) – Acids can be either strong or weak electrolytes May cause indicators to change colour Acids can react with metals to produce hydrogen gas (H 2 ) Acids can react with bases to form water and a salt Acids can also react with hydrogen carbonates to produce carbon dioxide, water and a salt

Properties of Bases Most of the time, we do not eat anything basic - they are too bitter (soap) They can be compared to acids by their properties – They are electrolytes that can change the colour of an indicator They feel slippery when wet When a base that contains hydroxide reacts with an acid, water and a salt form

Arrhenius Acids and Bases Although not entirely comprehensive, Arrhenius was one of the first scientists to propose general rules for acids and bases – He proposed acids to be those substances that produced H + in solution (dissolved in water) Bases were substances that produced OH - ions in solution (dissolved in water)

Arrhenius Acids When one hydrogen ionizes in solution, the acid is said to be monoprotic (HCl) Two hydrogen ions = diprotic (H 2 SO 4 ) Three H + = triprotic (H 3 PO 4 ) We now know that not all acids release hydrogen ions in solution Only hydrogen involved in strong polar bonds (large differences in electronegativities) with an anion will ionize

Difference between strong and weak bonds A strong acid (HCl) ionizes completely– HCl (aq) → H + (aq) + Cl - (aq) EN difference = 1.0 Even though a substance contains hydrogen, it may not be considered an acid if it does not ionize due to weak covalent bonds – CH 4(g) = EN difference = 0.3

Arrhenius Bases Group 1 metals react with water to produce solutions that are basic Ex. Na + H 2 O → NaOH + H 2 -the sodium hydroxide produced is basic -it is very soluble in water -as a result, in solution, it dissociates to form Na + and OH - -There are strong bases and weak bases due to differences in solubility

Practice 1. Write balanced equations for the reactions between the following: a.Aluminum and sulfuric acid b.Caclium carbonate and hydrobromic acid (metal carbonates produce water and carbon dioxide as products) 2. Write the net ionic equation for the reaction in question 1. (b)

Bronsted-Lowry Acids and Bases The definition of an Arrhenius acid and base does not include all acids and bases Bronsted-Lowry attempts to be more comprehensive Acids are defined as a H + donor and bases are H + acceptors Ex. Ammonia (NH 3 ) accepts a H + to form ammonium (NH 4 + ), so is a base

Conjugate Acids and Bases The particle formed by accepting the H + is called the conjugate acid Conversely, the particle that remains once the acid has donated a H + is called the conjugate base Ex. NH 3 + H 2 O → NH OH - – NH 3 = base (accepts H+), H 2 O = acid (donates H+), NH 4 + = conjugate acid (gains H+), OH - = conjugate base (loses H+). – Together, a conjugate acid-base pair forms – two substances linked by a single H +

Hydronium Ion (H 3 O + ) When water gains a hydrogen ion, it becomes positively charged H 2 SO 4 + H 2 O → H 3 O + + HSO 4 - However, water does not always do this… Sometimes, it acts as an acid, donating H + instead of gaining

Practice 1. Identify the conjugate acid-base pairs in each reaction. a. NH 4 + (aq) + OH - (aq) ↔ NH 3(aq) + H 2 O (l) b. HBr (aq) + H 2 O (l) ↔ H 3 O + (aq) + Br - (aq) c. CO 3 2- (aq) + H 2 O (l) ↔ HCO - (aq) + OH - (aq) 2. The products of an acid-base reaction are H 3 O + and SO Write a balanced equation for the reaction and identify the conjugate acid-base pairs.

Lewis Acids and Bases As an extension of his theory using electron dot diagrams, Lewis proposed that acids are electron pair acceptors and bases were electron pair donors – Such electrons are used to create covalent bonds between two atoms – H + can accept electron pairs so it can be classified as a Lewis acid

For the remainder of class… Identify each reactant as an acid or base. Which definition should you use to help with your classification? a.KOH + HBr → KBr + H 2 O b.HCl + H 2 O → H 3 O + + Cl - Q. 1-8, p. 593 Guided Reading for section 19.1